I mostly make a new PF, unless I want to have the same distribution as an earler made PF has. I exclusively work in the node network, and rightclick, etc. to make a new shader. Then just pull its output to the mask input where needed, and adjust settings of the PF. If you want to check the distribution, I always have a surface layer at the end of the line (first thing I do, and testcolor on), and use that to test the PF, and adjust scales, noise specifics, contrast, etc. to suit. Before rendering obviously disable the testlayer.
Nice composition and great scene, btw. In this case, grouping the stones by PF would perhaps be interesting...